Signals transmitted from Global Navigation Satellite System (GNSS) satellites consist of a carrier frequency modulated by a pseudo-random noise (PRN) code that is unique to each satellite.
All the satellites transmit at the same carrier frequency, but due to the high velocity of the satellites the signals will experience a Doppler shift in frequency before reaching the GNSS receiver. The Doppler shift can be several kHz in magnitude.
The pseudo-random noise (PRN) code sequence is 1023 units or chips long and repeats itself continuously. The code phase at the receiver at any given time is dependent on the distance between the receiver and the satellite.
There are several ways of acquiring signals from GNSS satellites. All these methods rely on the effect of autocorrelation, wherein a GNSS receiver will generate exact replicas of a carrier frequency and pseudo-random noise code and multiply these replicas with the incoming signal. If the carrier frequency and code phase of the generated signals match the ones in the incoming signal, it will produce maximum correlation power and the resulting mixed signal is easily detectable.
A straightforward way of acquiring the signals is to do a serial search, i.e. testing with all possible frequencies and code phases. In this case the total number of combinations is over 40,000 for each satellite signal (1023 code phases and 40 frequencies).
Most of the navigation and positioning devices are portable which means that low power consumption is a significant benefit. Signal acquisition and tracking require high amount of calculations which consumes power in a CPU. Any modification that reduces the amount of needed operations and leads to lower power consumption is an improvement to battery life.